The present invention pertains broadly to electronic control circuits for internal combustion engines and, more particularly, to a novel circuit for generating a sawtooth engine crank angle signal and an analog engine speed signal.
In controlling the operation of an internal combustion engine (for example, controlling the engine spark timing) it is often necessary to generate a sawtooth signal representative of engine crank angle as well as an analog signal representative of engine speed. An example of a circuit for generating these two signals is disclosed and claimed in U.S. patent application Ser. No. 388,673 filed Aug. 15, 1973 now U.S. Pat. No. 3,885,534, and assigned to the same assignee as the present application. In that application, there is disclosed an integrator circuit which is periodically reset at predetermined engine crank angles to thereby develop a sawtooth waveform representative of engine crank angle and at a frequency representative of the speed of the engine. There is further disclosed in that application a comparator circuit and an adjustable current source which are connected with the integrator circuit in a closed feedback loop to cause the peak value of the integrator sawtooth waveform to be regulated to a desired magnitude over a range of engine speeds. The operation of this circuit may be briefly summarized by saying that the comparator circuit compares the peak value of the integrator sawtooth just prior to resetting thereof against a reference signal and in accordance with the result of the comparison causes the adjustable current source to supply current calculated to cause the peak of the sawtooth to be regulated to a fixed magnitude over a range of engine operating speeds. In this way, the magnitude of the sawtooth at any given instant of time accurately reflects the correct engine crank angle. Current from the adjustable current source is passed through a resistor to develop an analog voltage signal representative of the speed of the engine.
One feature of the present invention is that both a sawtooth engine crank angle signal and an analog engine speed signal can be developed by using only two operational amplifiers in conjunction with a small number of passive circuit components. This is important in simplifying construction of this type of circuit for production application. Another feature of the present invention is the provision of an adjustable calibrating resistor in the integrator circuit which enables the magnitude of the analog engine speed signal to be more accurately related to the actual engine speed as established by the frequency of the sawtooth signal. Additional features, advantages, and benefits of the invention along with those already enumerated, will be seen in the ensuing description and claims which are to be taken in conjunction with accompanying drawing.